From: Alex Goins <agoins@nvidia.com>
To: "Christian König" <christian.koenig@amd.com>
Cc: John Hubbard <jhubbard@nvidia.com>,
dri-devel@lists.freedesktop.org, linux-mm@kvack.org,
amd-gfx@lists.freedesktop.org, Zi Yan <ziy@nvidia.com>
Subject: [PATCH RFC 0/1] drm/ttm: Allocate transparent huge pages without clearing __GFP_COMP
Date: Wed, 30 Sep 2020 17:18:20 -0500 [thread overview]
Message-ID: <20200930221821.13719-1-agoins@nvidia.com> (raw)
Hi Christian,
I've been looking into the DMA-BUFs exported from AMDGPU / TTM. Would
you mind giving some input on this?
I noticed that your changes implementing transparent huge page support
in TTM are allocating them as non-compound. I understand that using
multiorder non-compound pages is common in device drivers, but I think
this can cause a problem when these pages are exported to other drivers.
It's possible for other drivers to access the DMA-BUF's pages via
gem_prime_import_sg_table(), but without context from TTM, it's
impossible for the importing driver to make sense of them; they simply
appear as individual pages, with only the first page having a non-zero
refcount. Making TTM's THP allocations compound puts them more in line
with the standard definition of a THP, and allows DMA-BUF-importing
drivers to make sense of the pages within.
I would like to propose making these allocations compound, but based on
patch history, it looks like the decision to make them non-compound was
intentional, as there were difficulties figuring out how to map them
into CPU page tables. I did some cursory testing with compound THPs, and
nothing seems obviously broken. I was also able to map compound THP
DMA-BUFs into userspace without issue, and access their contents. Are
you aware of any other potential consequences?
Commit 5c42c64f7d54 ("drm/ttm: fix the fix for huge compound pages") should
probably also be reverted if this is applied.
Thanks,
Alex
Alex Goins (1):
drm-ttm: Allocate compound transparent huge pages
drivers/gpu/drm/ttm/ttm_page_alloc.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
--
2.25.1
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next reply other threads:[~2020-09-30 22:18 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-30 22:18 Alex Goins [this message]
2020-09-30 22:18 ` [PATCH RFC 1/1] drm-ttm: Allocate transparent huge pages without clearing __GFP_COMP Alex Goins
2020-10-01 7:19 ` [PATCH RFC 0/1] drm/ttm: " Christian König
2020-10-01 18:45 ` Alex Goins
2020-10-02 6:41 ` Christian König
2020-10-02 7:31 ` Daniel Vetter
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